1. Field of the Invention
The present invention relates to material shredders and particularly to a shredder for reducing rubber tires and similar bulk materials into small pieces.
2. Description of the Prior Art
The disposal of worn-out rubber tires is a major enrivonmental problem. When reduced to particulate form, the tires can be more readily disposed of than otherwise by using the particles either as landfill, as a constituent of paving materials, or as a source of heat energy.
Prior shredders suitable for shredding rubber tires and similar bulk scrap materials have been of two general types.
One such type, exemplified by Ehrlich U.S. Pat. No. 3,561,308, employs a series of reciprocating or oscillating knives which coact with a stationary anvil to shear a tire moving across the anvil into diamond-shaped particles. This type of tire shredder, although effective, is too slow to be economical for most commercial high-production applications, requires a complex knife-driving mechanism, and has knives which are difficult and time consuming to resharpen.
The other general type of tire shredder is characterized by opposed counterrotating drums, with each drum defining a series of axially spaced-apart cutting discs. In most shredders of this type, as exemplified by Schwarz U.S. Pat. No. 3,880,361, and Holman U.S. Pat. No. 3,931,935, the discs of one drum extend into the spaces between discs of the other drum and the discs have peripheral cutting teeth so that the overlapping discs interact to cut or tear tires into irregular pieces. The cutting teeth must be kept sharp to maintain the cutting effectiveness of the shredders. Additional shredders of this general type are shown, for example, in U.S. Pat. Nos. 3,664,592 and 3,845,907, as well as numerous patents showing similar types of apparatus in the crushing and grinding machine field.
In another counterrotating drum-type shredder, discs on one drum directly oppose discs on the opposite drum. One of each pair of opposed discs tapers to a single continuous knife edge at its outer periphery and abuts a blunt outer peripheral surface of the opposed disc. A shredder of this type is shown in Krigbaum U.S. Pat. No. 3,817,463. Material fed between the opposed discs is shred into long strips. In a modification of this concept, shown in Krigbaum U.S. Pat. No. 3,727,850, thin knife-edged discs spaced along one counterrotating drum extend into abutment with the blunt bottom surfaces of opposed grooves in the other drum to achieve a similar slicing action. In either case, it appears that a knife edge must be maintained on the cutting discs to achieve an effective cutting or slicing action. Such edge would rapidly dull in use through actual contact between the cutting elements of opposed drums, necessitating frequent resharpening and replacement of the cutting discs. Also, at least two sets of vertically stacked drums must be used, with the strips from the upper set being fed into the lower set for reshredding in order to reduce the strips to an acceptable small size.
Paper shredders have used the counterrotating drumoverlapping disc principle, as exemplified by Antonsen U.S. Pat. No. 1,939,246 and Goldhammer U.S. Pat. No. 3,630,460. The overlapping cylindrical discs of such paper shredders are typically relatively widely spaced apart so as to shred paper fed therebetween with a tearing rather than a shear-cutting action. Other paper shredders of this type have their overlapping discs in actual sliding frictional contact to cut the paper into strips, requiring considerable power to drive them. In either case, such paper shredders are typically small office machines and have not been thought applicable to the shredding of thick, tough, bulk elastic materials such as rubber tires.
The aforementioned Holman patent also discloses a tire shredder similar to the devices of the paper shredder patents in operating principle. However, the Holman shredder has no effective means for recirculating shredded scraps through the shredder to ensure that the end product is of a desired small size suitable for use as fuel chips or fill material.
From the foregoing it will be apparent that there is a need for a tire shredder capable of shredding tires into small particles at a high rate and having low and easy maintenance characteristics.